1. Field of the Invention
The present invention relates to a method of adjusting a modulation accuracy whereby, in a digital modulator used in a mobile communication such as the PDC, PHC (personal handy phone system), and the like, the modulation accuracy deteriorated from a phase error of a local signal applied to an orthogonal modulator being a component of the digital modulator is corrected without using a special hardware, and an apparatus of the same.
2. Description of the Related Art
FIG. 4 is a block diagram showing a construction of a conventional digital modulator. In the conventional example shown in FIG. 4, the modulator employs the orthogonal modulation circuit to meet the digital mobile communication system using a modulation system such as the .pi./4 shift QPSK (quadriphase shift keying), GMS (Gaussian filtered minimum shift keying), or 16 QAM (quadrature amplitude modulation).
The digital modulator in FIG. 4 will now be described.
A modulation data 1A in a digital data train is a data to conform the signal format standardized by the communication system for the PDC, PHS, and the like, which is a digital data train expressed by the logic "1" and "0".
The modulation data 1A is converted into an I signal 1B and Q signal 1C through an IQ converter 12. The conversion rule to convert the modulation data 1A into the I signal 1B and Q signal 1C is determined in accordance with the modulation system such as the .pi./4 shift QPSK.
A modulating local signal 1D being a non-modulated sinewave, on the other hand, is multiplied by the foregoing I signal 1B in a multiplier 16 to produce an I modulation signal 1E, and is at the same time supplied to a phase converter 18 to produce a phase conversion signal 1G with a phase shifted by 90.degree.. The phase conversion signal 1G is multiplied by the foregoing Q signal 1C in a multiplier 17 to produce a Q modulation signal 1F.
An adder 19 adds the I modulation signal 1E and the Q modulation signal 1F to produce a modulation signal 1H as an output from a modulator 20.
The multiplier 16, multiplier 17, phase converter 18, and adder 19 constitutes an orthogonal modulator 11.
And, for the modulating local signal 1D, a sinewave signal in a medium frequency range from 100 MHz to 200 MHz is generally used, and the modulation signal 1H is also a modulated signal in the same frequency range.
Incidentally, the phase conversion signal 1G is required to have the accurate 90.degree. phase shift to the modulating local signal 1D. However in reality, due to dispersion of the circuit elements and wiling length of the phase converter 18, the accurate 90.degree. phase difference is not necessarily secured and errors are included therein. And, if there is an error from 90.degree. in the phase difference between the phase conversion signal 1G and the modulating local signal 1D, the error will deteriorate the modulation accuracy of the modulation signal 1H.
Next, the modulation accuracy will be described with reference to FIG. 5. For example, the .pi./4 shift QPSK modulation system determines four values ("00", "01", "11", "10") in accordance with phase shifts and transmits data. The modulation accuracy is expressed by the effective value of an error between vectors at a signal point. In FIG. 5, a vector error .DELTA.q between an ideal vector A and an actual vector B represents the modulation accuracy.
As described above, the phase conversion signal 1G in FIG. 4 does not necessarily hold the accurate 90.degree. phase difference to the modulating local signal 1D due to dispersion of the circuit elements and wiling length.
Next, the influence on the modulation accuracy when the 90.degree. phase difference is not obtained between the phase conversion signal 1G and the modulating local signal 1D will be described. When the accurate 90.degree. phase difference is not obtained between the I modulation signal 1E and the Q modulation signal 1F, the Q axis is shifted to a Q1 axis and a component D on the Q axis is given by a component D1 on the Q1 axis as shown in FIG. 6.
Here, an error between an ideal vector A produced by the vector sum of the component D on the Q axis and a component E on the I axis and a vector C produced by the vector sum of the component D1 on the Q1 axis and the component E on the I axis will give a degradation on the modulation accuracy.
Consequently, in the orthogonal modulator for the digital signal, it is necessary to set the phase difference between the I modulation signal and the Q modulation signal accurately to 90.degree.. In the conventional exercise, this phase difference is adjusted by means of a variable resistor or variable capacitor, which requires extra circuits and extra adjustment time.